Department of Chemistry and Biochemistry, The Ohio State University , 100 West 18th Avenue, Columbus, Ohio 43210, United States.
Inorg Chem. 2013 Oct 7;52(19):11302-10. doi: 10.1021/ic401559b. Epub 2013 Sep 25.
The complexes (BDI)MgX(THF), where X = Bu(n), NEt2, and OBu(t), are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu(n), ΔH(#) (kcal mol(-1)) = 13.4 ± 0.4 and ΔS(#) (cal mol(-1) K(-1)) = 6.3 ± 1.6; X = NEt2, ΔH(#) (kcal mol(-1)) = 15.2 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 11.4 ± 2.3; X = OBu(t), ΔH(#) (kcal mol(-1)) = 16.4 ± 0.3 and ΔS(#) (cal mol(-1) K(-1)) = 9.5 ± 1.3. The apparent aryl group rotations involving the BDI ligands have, within experimental error, the same activation parameters as the THF dissociation, which suggests that the two are correlated involving a three coordinate reactive intermediate akin to what is well-known for related (BDI)ZnR compounds involving three-coordinate trigonal planar Zn(2+). At higher concentrations of THF for X = Bu(n) and OBu(t), but not for X = NEt2, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-d8, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the Ia mechanism to have activation parameters: ΔH(#) (kcal mol(-1)) = 5.4 ± 0.1 and ΔS(#) (cal mol(-1) K(-1)) = -20.9 ± 0.3 for X = Bu(n) and ΔH(#) (kcal mol(-1)) = 8.3 ± 0.8 and ΔS(#) (cal mol(-1) K(-1)) = -19.8 ± 3.0 for X = OBu(t). For the complex (BDI)MgBu(n)(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs more readily than for its THF analogue, as expected for the more sterically demanding Lewis base O-donor: ΔH(#) (kcal mol(-1)) = 12.8 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 8.6 ± 1.8. At very low temperatures in toluene-d8, restricted rotation about the Mg-O(THF) bond is observed for the complexes where X = NEt2 and OBu(t) but not for the complex where X = Bu(n). These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.
（BDI）MgX（THF）配合物中的 THF 交换反应：通过离解机制进行的低浓度添加下的反应
（BDI）MgX（THF）配合物（其中 X = Bu（n）、NEt2 和 OBu（t）），通过离解机制在低浓度添加 THF 时发生 THF 交换：X = Bu（n），ΔH（#）（kcal mol-1）= 13.4 ± 0.4，ΔS（#）（cal mol-1 K-1）= 6.3 ± 1.6；X = NEt2，ΔH（#）（kcal mol-1）= 15.2 ± 0.5，ΔS（#）（cal mol-1 K-1）= 11.4 ± 2.3；X = OBu（t），ΔH（#）（kcal mol-1）= 16.4 ± 0.3，ΔS（#）（cal mol-1 K-1）= 9.5 ± 1.3。实验误差范围内，涉及 BDI 配体的芳基基团旋转的表观活化参数与 THF 解离的活化参数相同，这表明两者相关，涉及类似于相关（BDI）ZnR 化合物中众所周知的三配位反应性中间体的三配位三角平面 Zn（2+）。在 X = Bu（n）和 OBu（t）的较高 THF 浓度下，但在 X = NEt2 时则不然，在甲苯-d8 中，芳基基团旋转的表观缔合温度从纯化合物的缔合温度降低，并且提出了证据表明，这与缔合交换过程相关，在纯 THF 中该过程占主导地位。我们估计 Ia 机制的活化参数为：X = Bu（n）时，ΔH（#）（kcal mol-1）= 5.4 ± 0.1，ΔS（#）（cal mol-1 K-1）= -20.9 ± 0.3，X = OBu（t）时，ΔH（#）（kcal mol-1）= 8.3 ± 0.8，ΔS（#）（cal mol-1 K-1）= -19.8 ± 3.0。对于（BDI）MgBu（n）（2-MeTHF）配合物，与添加的 2-MeTHF 的离解交换比其 THF 类似物更容易发生，这是预料之中的，因为路易斯碱 O-供体的空间要求更高：ΔH（#）（kcal mol-1）= 12.8 ± 0.5，ΔS（#）（cal mol-1 K-1）= 8.6 ± 1.8。在甲苯-d8 中的极低温度下，观察到 X = NEt2 和 OBu（t）的配合物中 Mg-O（THF）键的限制旋转，但 X = Bu（n）的配合物则不然。这些观察结果是通过动态 NMR 研究确定的，与这些配合物在溶液中的反应性相关。
